Conventionally, molded plastic optical parts are limited to small lens and flat sheet applications. Molded arbitrary surface plastic parts are not used for optical applications because it is very difficult to achieve the dimensional control needed for optical applications. For typical optical applications, dimensional control of 0.05% is needed. For precision optical applications, dimensional control of 0.005% is needed.
Conventionally, mirrors are made from glass that has been coated with a thin layer of aluminum or silver. When used in a rear projection television (RPTV) or other optical system, mirrors are often coated on the reflective or first surface side. This avoids secondary and tertiary reflections caused by the glass/air interface. These reflections can cause ghost images and destroy the fidelity of the mirror. Glass is generally used as the substrate because it can be made very flat, is chemically inert, and thermally stable. The flatness is important for accurate one-to-one reproduction of the reflected image. The chemical inertness is important because you do not want an adverse chemical reaction between the aluminum or silver coating and the mirror substrate. This is particularly important with silver, which is the most reflective, but also can be easily damaged by chemicals migrating out of the substrate. Thermal stability is important because the coating process is done at high temperatures.
Most first surface mirrors are plated in a vacuum chamber where the aluminum is deposited in a very thin film, typically only a few micro-inches thick. This makes a very reflective and smooth surface, as flat and smooth as the glass substrate itself.
In various applications there is a need for mirrors that have different surface shapes. Mirrors that have an inward curve or an outward curve are useful. Mirrors that have curves in both directions or even arbitrary curves are also useful in some optical applications. In rear projection TV application, it is desirable to make the optical projection angle steeper in order to make the product cabinet dimension thinner. The arbitrary curve mirror is used to counteract the trapezoid geometry shape problem introduced by the steeper projection angle. This class of mirrors is often described by polynomials and can be quite complex in their surface shapes. Glass is not suitable for mass manufacturing these difficult shapes because of the difficulty forming the material.
Plastics are better suited for forming arbitrarily shapes surfaces and are cheaper and lighter in weight than optical glass. However, plastics are not thermally stable and contain organic compounds that tend to leach into the aluminum or silver plating and ruin the reflectivity. For these reasons they have not been used previously for high volume commercial first surface mirrors.
Conventionally, resin injection mold tools for forming plastic parts are Class A molds and are made out of high quality hardened steel, and can achieve dimensional tolerance well under 0.005%.
However, formed resin parts may have a shrinkage factor of about 0.5 to about 4%, the majority of the shrinkage occurs in the mold tool, as the resin cools from a temperature above its melting point, to a temperature below its glass transition temperature. About 15-30% of the shrinkage occurs after the resin part is ejected from the mold and allowed to cool to the room temperature. For cost and capacity throughput reasons, often the injected part is not allowed to cool down to room temperature in the mold.
Typical plastic resin part warpage from molding can be controlled to about +/−0.5% if the molding condition is well controlled. However, +/−0.5% is still an order of magnitude higher than the requirement for optical applications. For the most part, it is differential shrinkage of the injected part within the mold and without the mold that causes the part to deform and deviate from the tight tolerance of the mold.
The warpage is typically caused by differential cooling when the part is ejected from the tool. The thin edge(s) of a typical optical part are exposed to ambient air in 3 sides and can cool relatively faster than the center of the optical element. Even though the plastic resin is below its glass transition temperature, differential cooling still causes differential stress to build up and deform the shape beyond the range needed for optical application.
As such a new resin part and method of manufacture having less warpage are required.